According to Danbury News-Times report, the explosion took place in 620-megawatt Kleen Energy plant which was still under construction.
There were about 100 people inside who were said to be testing some recently-completed upgrades to the facility.
Sebastian Giuliano, the mayor of Middletown in Connecticut, said, "We know that 12 individuals have been injured. Five individuals are known to have lost their lives."
"There could be anywhere from 100 to 200 people working on the site on any given day. Exactly what's that number, that's the starting point and that's the number they can't nail down today," he was quoted by news agency as saying.